1. Field of the Invention
The inventions relate generally to optical communications and, more particularly, to optical communications over multiple wavelength-channels.
2. Discussion of the Related Art
This section introduces aspects that may be helpful to facilitating a better understanding of aspects of the invention. Accordingly, the statements of this section are to be read in this light. The statements of this section are not admissions about the scope of the prior art.
In a dense wavelength division multiplexed (DWDM) system, it is often desirable be able to add or drop any desired wavelength-channel at access points of the DWDM system. An optical component for adding or dropping any wavelength-channel at its optical ports is known as a colorless optical add/drop module.
FIG. 1 shows an exemplary optical add/drop module 10. When functioning to drop one or more wavelength-channels, the optical add/drop module 10 receives optical signals at external optical port 12 and routes the received optical signals towards eight external optical ports 141, 142, 143, 144, 145, 146, 147, 148. When functioning to add one or more wavelength-channels, the optical add/drop module 10 receives optical signals at the eight external optical ports 141, 142, 143, 144, 145, 146, 147, 148 and routes the received optical signals to the external optical port 12.
The optical add/drop module 10 includes an array 18 of seven 1×2 optical intensity splitters 161, 162, 163, 164, 165, 166, 167 and eight tunable optical band pass filters 201, 202, 203, 204, 205, 206, 207, 208. The array 18 has a tree-structure with three layers. In the first layer, an optical input (OI) of the optical intensity splitter 161 connects to the external optical port 12 of the optical add/drop module 10. In the second and third layers, OIs of the layers connect via optical waveguides (OWs) to optical outputs (OOs) of the previous layer. In the third layer, OOs of the optical amplitude splitters 164–167 connect via OWs to OIs of corresponding tunable optical band pass filters 201–208. The tunable optical band pass filters 201–208 have OOs that function as the external optical ports 141–148 of the optical add/drop module 10.
In the array 18, each 1×2 optical intensity splitter 161–167 directs about half of the light intensity received at its OI to each of its OOs. In particular, this splitting of received light intensities is performed in non-wavelength selective manner so that light of each wavelength-channel is directed to both OOs of the 1×2 optical intensity splitters 161–167. For that reason, the array 18 redirects only about an eighth of the received light on any wavelength-channel to the tunable optical band pass filters 201–208.
Each tunable optical band pass filter 201–208 has a tunable center band pass wavelength. In particular, the band passes of the individual optical band pass filters 201–208 may be separately tuned to selectively pass any one of eight preselected adjacent wavelength-channels and to selectively stop the remaining seven of the preselected adjacent wavelength-channels. For that reason, the optical add/drop module 10 is able to function as a colorless optical module for dropping wavelength-channels to the external optical ports 141–148.
The optical add/drop module 10 is also a colorless add module when the external optical ports 141–148 function as optical input ports and the external optical port 12 functions as an optical output port.
Unfortunately, the 1×8 optical add/drop module 10 is wasteful with light received at external optical port 12. In particular, the optical add/drop module 10 only delivers about one eighth of such received light of any wavelength-channel to the external optical ports 141–148. Furthermore, in going from the external optical port 12 to any of the external optical ports 141–148, light is transmitted through a long chain of optical components. In each such chain, optical losses can further reduce the intensity of the delivered light.